[DOC] Update install instructions in docs (#7572)

I noticed the install commands on [the
docs](https://triton-lang.org/main/getting-started/installation.html#python-package)
are outdated (executing them as-is gives `triton/python does not appear
to be a Python project: neither 'setup.py' nor 'pyproject.toml'
found.`).

This PR updates those instructions to match those currently in the
readme, specifically:
- Install from `pip install -e python`  to `pip install -e .`
- Wheels from `CPython 3.8-3.12 and PyPy 3.8-3.9.` to `CPython
3.9-3.13.` (match readme)
- Removes the nightly installation instruction
- Tests to the new `make dev-install; make test; make test-nogpu` setup

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- Select one of the following.
  - [ ] I have added tests.
    - `/test` for `lit` tests
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installation instructions in the docs`.

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Author: klamike

docs/getting-started/installation.rst

@@ -14,14 +14,7 @@ You can install the latest stable release of Triton from pip:
 
       pip install triton
 
-Binary wheels are available for CPython 3.8-3.12 and PyPy 3.8-3.9.
-
-And the latest nightly release:
-
-.. code-block:: bash
-
-      pip install -U --index-url https://aiinfra.pkgs.visualstudio.com/PublicPackages/_packaging/Triton-Nightly/pypi/simple/ triton-nightly
-
+Binary wheels are available for CPython 3.9-3.13.
 
 -----------
 From Source
@@ -35,25 +28,25 @@ You can install the Python package from source by running the following commands
 
 .. code-block:: bash
 
-      git clone https://github.com/triton-lang/triton.git;
-      cd triton/python;
-      pip install ninja cmake wheel; # build-time dependencies
+      git clone https://github.com/triton-lang/triton.git
+      cd triton
+
+      pip install -r python/requirements.txt # build-time dependencies
       pip install -e .
 
 Note that, if llvm is not present on your system, the setup.py script will download the official LLVM static libraries and link against that.
 
 For building with a custom LLVM, review the `Building with a custom LLVM <https://github.com/triton-lang/triton?tab=readme-ov-file#building-with-a-custom-llvm>`_ section on Github.
 
-You can then test your installation by running the unit tests:
+You can then test your installation by running the tests:
 
 .. code-block:: bash
 
-      pip install -e '.[tests]'
-      pytest -vs test/unit/
+      # One-time setup
+      make dev-install
 
-and the benchmarks
-
-.. code-block:: bash
+      # To run all tests (requires a GPU)
+      make test
 
-      cd bench
-      python -m run --with-plots --result-dir /tmp/triton-bench
+      # Or, to run tests without a GPU
+      make test-nogpu